1. Field of the Invention
The present invention relates to an ultrasonic vibration driven motor. More particularly, the invention relates to an ultrasonic vibration driven motor in which, when electrical energy is fed to piezoelectric elements or other electromechanical energy conversion elements of the motor, a vibration member of the motor, for example a bar or pencil type vibration motor, which clamps the electromechanical energy conversion elements from the lateral sides of the conversion elements, bends and vibrates, so that surface portions of the vibration member make a circular or elliptical movement, whereby a moving member pressed against the vibration member is driven by friction.
2. Related Background Art
In a vibration member of a bar-type ultrasonic motor, as shown in FIGS. 2 and 3, two pairs of phase-A and phase-B driving piezoelectric elements (hereafter, PZT) 1 and 2, and a sensor PZT 3 are clamped between vibration member structures 4 and 5, (made of an iron, copper, aluminum, or other metal having a low damping characteristic), and secured with a tightening bolt (not shown). Thus, the vibration member is axially symmetrical as a whole, and has degeneracy in which two transverse bending vibration modes have the same shape and same undamped natural frequency. Reference numerals d1 to d6 denote electrode disks. An a.sub.1 sin .omega.t driving signal is fed to the phase-A PZT 1, and an a.sub.2 cos .omega.t driving voltage is applied to the phase-B PZT 2.
Therefore, as a positional phase difference between two pairs of phases A and B is held at 90.degree., vibrations develop in the direction of an excitation force applied to PZTs. That is to say, two bending vibrations develop with a positional phase difference of 90.degree..
As a temporal phase difference between AC fields applied to these driving PZTs is held at 90.degree., a temporal phase difference between bending vibrations is retained around 90.degree.. When the strengths of the AC fields applied to the PZTs are adjusted, the bending vibrations will have the same amplitude.
As a result, portions surface of the vibrating member make a circular movement within a plane transverse to the axis. When a moving member (not shown) is pressed on the vibrating member to rub against these portions, the moving member is thus frictionally driven.
The bar-type ultrasonic vibration driven motor is shaped axially symmetrically. However, a machining error or presence of screws prevents the motor from having a perfectly axially symmetrical shape. Due to influence of heterogeneous materials or irregular clamping pressure, the degeneracy of modes is destroyed, the directions of transverse natural vibration modes are determined uniquely, and the undamped natural frequencies of the natural vibration modes differ from each other.
Since the causes of the foregoing problems are uncertain, the directions of the natural vibration modes cannot be detected in advance.
Due to the aforesaid causes, the directions of arranging the driving PZTs differ from the unique directions of the natural vibration modes. This poses the problems below.
As shown in FIG. 4, assume that the directions of the natural modes, A.OMEGA. and B.OMEGA., deviate by .DELTA..OMEGA. from the directions of phase-A and phase-B PZTs, A and B to be arranged mutually transversely.
The phase-A PZTs excite two standing waves of vibrations in the A.OMEGA. and B.OMEGA. directions according to the deviation .DELTA..OMEGA..
A response phase of a displacement deriving from an excitation force, as shown in FIGS. 5A and 5B, varies greatly with frequencies around an undamped natural frequency. Therefore, when the undamped natural frequencies in the A.OMEGA. and B.OMEGA. directions differ from each other, as shown in FIG. 5A, the bi-directional vibration phases excited by the phase-A PZTs 1 differ from each other. The phase difference is represented as .DELTA..phi..
Assuming that an A.OMEGA. direction component of an excited vibration displacement is a.times.sin (.omega.t), a B.OMEGA. direction component is provided as b.times.sin (.omega.t+.DELTA..phi.).
On the other hand, when an AC field shifted by a phase, .DELTA..psi., from an AC field applied to the phase-A PZTs 1 is applied to the phase-B PZTs 2, bi-directional vibrations (standing waves) are excited. The vibrations in the A.OMEGA. and B.OMEGA. directions are provided as b.times.sin (.omega.t+.DELTA..psi.) and a.times.sin (.omega.t+.DELTA..phi.+.DELTA..psi.) respectively, where a:b=cos.DELTA..OMEGA.:sin .DELTA..OMEGA..
In the foregoing situation; that is, when the electrode patterns of the PZTs differ from the directions of the natural vibration modes, if the phase difference .DELTA..psi. is set to an appropriate value, then the surface portions of the vibration member make a circular movement. However, .DELTA..phi. or .DELTA..OMEGA. has a nature of varying depending on a motor. It is, therefore, impossible to set .DELTA..psi. to a value causing the circular movement in practice.